One example of the prior art of this type of working machine is disclosed in Japanese Patent Laid-Open (KOKAI) No. 56-150235.
The conventional working machine is, as shown in FIGS. 1 and 2, arranged such that the proximal end portion of a first boom cylinder 3 is pivotably supported on a bell crank 4 that is pivotably supported on a chassis 1 through a bracket 2, and the bell crank 4 is driven by a bell crank cylinder 5, thereby folding the working machine in a compact form. In other words, in order to pivot the proximal end portion of the first boom cylinder 3 in such a manner that no dead point will occur in the link mechanism, it is necessary to provide not only the bell crank 4 and the bell crank cylinder 5 but also a hydraulic circuit means, including a valve and other elements, for controlling the bell crank cylinder 5. Accordingly, the number of parts increases and the structure becomes complicated, and the production cost also rises. In addition, the operation of the working machine also becomes complicated, and a failure is likely to occur. Thus, the prior art involves various problems.
It should be noted that the above-mentioned dead point is a position of the link mechanism in which the pivot point at which the first boom member and the distal end portion of the first boom cylinder are connected pivotably to each other, the pivot point at which the proximal end portion of the first boom member is pivotably attached to the chassis and the pivot point at which the proximal end portion of the first boom cylinder is pivotably attached to the chassis align with each other in a straight line during a pivoting motion of the first boom cylinder. When this position occurs, no hydraulic pressure from the first boom cylinder acts to the outside. In other words, the first boom cylinder cannot function as a driving member. In this application, the term "dead point" is used in the sense of the above.